332 AMERICAN SPIDERS AND THEIR SPINNINGWORK. 



imrts of the abdomen are distended during gestation, and this distension 

 produces changes in tlie color of the animal, it may be by modifying in 

 some way the various secretions from the liver and other organs, and in 

 some cases, perhaps, widening the intervals between color centres and color 

 hairs, and breaking up groups of the same. 



The little j^its or dark spots upon the dorsum of the abdomen, whicli 

 mark the attachment of the muscles within, seem to me to be centres for 

 the aggregation of coloring material. At least the dorsal pat- 

 Effects of terns appear to be grouped in some regular way around these 

 Action iiiuscular attachments. Thus the action of the muscles on the 

 skin and chitinous shell or walls seems to compel certain aggre- 

 gations along the lines of use that form these colors and patterns. It 

 might be important in this connection to consider what is the ordinary 

 effect of muscular action upon the distribution of pigment and colored 

 hairs in vertebrate animals. 



The color rings or annuli around the joints of the legs of spiders may 

 be influenced by action of the muscles. The tendency of the.se darker and 

 more vivid colors is towards the ends of the joints, as though Ijy the at- 

 tachments and iirevailing outward action of the muscles the pigment were 

 forced mechanically or otherwise attracted toward these points. The foot 

 or terminal joint is usually dark, and often black. In the cei^halothorax 

 may be noted the same tendency of color to group itself somewhat sym- 

 metrically around the jjoints of muscular attachment, particularly the cen- 

 tral depression. 



IV. 



Color and markings are undoubtedly influenced by sex. Peckham, after 

 summarizing the Attidae of France from the studies of M. Simon, finds 

 that in thirty-nine species the male is plainly unlike the female. 

 Influence i^pj^g {^^ twenty-six instances much more conspicuous, while in 

 „ , fifty-five the sexes are similar, or, if they differ, the male is no 



more conspicuous than the female. These facts make it clear 

 tliat the sexes commonly differ, the male being brighter tlian the female. 

 Peckham considers it not too much to say that in the Attidffi at least 

 two-fifths of all the species have tJie male more conspicuous than the 

 female. ' 



Menge, in referring to the greater brilliancy of the male of INIicromata 

 ornata, says that it only assumes its bright color as a bridal adornment, 

 and in this connection makes a statement that in the Thomisida; and Sal- 

 tigradas the males are generally more beautifully colored than the females.'^ 



Philseus militaris, a common American Attoid, is another illustration. 

 In the male the cephalothorax and abdomen are bright l)ronze brown, the 



' Sexual Selection, page 20. - Menge, Preussische Spinnen, II., page 396. 



